Study On The Preparation And Properties Of Barium Titanate-based Ferroelectric Thin Films

Due to its electrically controllable spontaneous polarization characteristics,ferroelectric thin films have shown great potential in multi-functionalization and new applications.Recently,the resistive switching(RS)effect and anomalous photovoltaic effect in ferroelectrics have attracted widespread attention,but the unclear microscopic conduction mechanism hinders their practical application and development.We comparatively studied the influence of doping Sr²⁺and Zr⁴⁺ions on the microstructure and electrical properties of barium titanate film,and further explored non-volatile resistive random-access memory and photovoltaic devices based on barium zirconate titanate(BaZr_xTi_1-xO₃,BZT)films.We paid attention to the relationship between the microstructure of the film and the macroscopic electrical properties of the device.Firstly,the solid-phase reaction method was used to prepare BZT targets with x values of 0,0.05,0.10,0.15 and 0.2.Secondly,the optimal process for growing BZT thin films by magnetron sputtering was systematically explored.The best parameters for growing BZT films are as follows:RF power is 100W,pressure is 1.5Pa,Ar:O₂ is 40:40,and the annealing temperature is 800°C.The prepared BZT films have well-developed crystal structures,good dielectric and ferroelectric properties.Subsequently,BZT films with different Zr/Ti and Ba_0.6Sr_0.4TiO₃ films were prepared by the optimal parameters.The results show that the lattice constants and optical band gap of the BZT film will increase with the increase of Zr⁴⁺content,while the polarization intensity and Curie temperature will decrease with the increase of Zr⁴⁺ content.In addition,the increase of Zr⁴⁺ content will improve the dielectric properties and ferroelectric fatigue properties of the film at room temperature.Sr²⁺ ions doping can also reduce the Curie temperature and band gap of the barium titanate film,while increasing the dielectric constant,but the dielectric loss at high frequencies is slightly larger than that of BaZr_0.2Ti_0.8O₃.Finally,the RS and photovoltaic performance of the BZT film were studied.The Cu/BZT/Pt films with different Zr/Ti ratios all have RS phenomena,but there are two different curves.One type is that the current of device will suddenly increase or decrease at a certain voltage,which occurs in the BaZr_xTi_1-xO₃ film with x?0.10.The other type is the current of device always changes continously,which occurs when the Zr⁴⁺ content is high.It is suggested that Zr⁴⁺ doping change the RS behavior by affecting the crystal structure and leakage current of the BZT film.When the Zr⁴⁺content is low,the stability of the crystal structure is poor,and the leakage current of the thin film is large,there are a large number of grain boundaries and defects in the film.It is easy to form a conductive channel in the film that makes the current suddenly increases,which can be explained by the oxygen vacancy conductive filament model.When the Zr⁴⁺concentration increases,the leakage current of the BZT film will be suppressed,which will increase the difficulty of forming conductive filaments.At this time,the RS behavior is mainly controlled by the defects and the ferroelectric polarization charges on the interface.In addition,the photovoltaic effect of the Cu/BZT/Pt thin film comes from the combined effect of the depolarization field and the Schottky barrier,and is also related to the change of crystal structure caused by Zr⁴⁺doping.When the Zr⁴⁺content is low,it is easy to form conductive channels in the film during the electrical stimulation to make the photovoltaic disappear.When the Zr⁴⁺ content is high,the polarization will affect the photovoltaic performance by modulating the net built-in electric field of the device.The magnitude of the photo-generated voltage is related to the direction of polarization.